1. Field of the Invention
The present invention relates to a hydrostatic transmission (hereinafter, referred to as “HST”) including a hydraulic pump, a hydraulic motor, a pair of main fluid passages interposed between the hydraulic pump and the hydraulic motor so as to constitute a closed fluid circuit, and charge check valves for supplying fluid to the main fluid passages of the closed fluid circuit.
2. Related Art
There is a well-known conventional transmission system referred to as a “hydraulic mechanical transmission (hereinafter, referred to as “HMT”)” that is a combination of an HST and a planetary gear mechanism. The HMT is used to output high power for driving a large vehicle, e.g., a tractor, a wheel loader and a truck. However, the main fluid passages of the HST are liable to leak fluid because they are subjected to high fluid pressures to ensure the high output power of the HMT. To avoid the fluid leak, the main fluid passages must have a high quality in fluidal tightness and in strength so as to withstand the high pressure of fluid, however, it is expensive to make such main fluid passages.
Then, the HST has been considered to have an effective means for recovering the fluid leak from the main fluid passages and for supplying the recovered fluid to the main fluid passages. A typical means is a charge pump driven together with a hydraulic pump of the HST by an engine. However, the charge pump has a problem that when a vehicle is parked on a slope while stopping an engine, fluid leaks from the main fluid passages rapidly and the pressure of fluid in the main fluid passages becomes insufficient to hold the stationary vehicle so that the parked vehicle may unexpectedly descend the slope because the charge pump cannot be driven by the engine to compensate for the lack of pressure of fluid in the main fluid passages.
Therefore, as disclosed by JP H7-158730 A, the HST may be provided with an orifice in addition to the charge pump. This orifice siphons fluid from a fluid sump into the main fluid passages because of hydraulic depression of the main fluid passages caused by rotation of a hydraulic motor of the HST following the descent of the vehicle. In this regard, the HST is provided with a pair of charge check valves each of which can be opened to allow fluid to flow into the corresponding main fluid passage when the corresponding main fluid passage is hydraulically depressed relative to a charge fluid passage on the upstream side of the charge check valve. The fluid siphoned by the orifice is supplied to the hydraulically depressed main fluid passage via the opened charge check valve.
However, the fluid supply by use of the orifice is still insufficient to ensure the high output power of the HMT. In this regard, each fluid passage between each charge check valve and each main fluid passage has a constant sectional area that cannot be increased to increase flow of fluid charged into the main fluid passage so as to compensate for the shortage of fluid siphoned by the orifice. As a result, the HST of the HMT may have a problem caused by the lack of hydraulic pressure in the main fluid passages, e.g., cavitation that damages related devices and causes noise.